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(#) Missing Backup Pin

!!! WARNING: Missing Backup Pin
   This is a warning.

Id
:   `MissingBackupPin`
Summary
:   Missing Backup Pin
Severity
:   Warning
Category
:   Correctness
Platform
:   Android
Vendor
:   Android Open Source Project
Feedback
:   https://issuetracker.google.com/issues/new?component=192708
Since
:   2.3.0 (March 2017)
Affects
:   Resource files
Editing
:   This check runs on the fly in the IDE editor
See
:   https://developer.android.com/preview/features/security-config.html
Implementation
:   [Source Code](https://cs.android.com/android-studio/platform/tools/base/+/mirror-goog-studio-main:lint/libs/lint-checks/src/main/java/com/android/tools/lint/checks/NetworkSecurityConfigDetector.java)
Tests
:   [Source Code](https://cs.android.com/android-studio/platform/tools/base/+/mirror-goog-studio-main:lint/libs/lint-tests/src/test/java/com/android/tools/lint/checks/NetworkSecurityConfigDetectorTest.java)

It is highly recommended to declare a backup `<pin>` element. Not having
a second pin defined can cause connection failures when the particular
site certificate is rotated and the app has not yet been updated.

(##) Example

Here is an example of lint warnings produced by this check:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~text
res/xml/network_config.xml:5:Warning: A backup <pin> declaration is
highly recommended [MissingBackupPin]
    &lt;pin-set&gt;
     -------
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Here is the source file referenced above:

`res/xml/network_config.xml`:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~xml linenumbers
&lt;?xml version="1.0" encoding="utf-8"?&gt;
&lt;network-security-config&gt;
    &lt;domain-config&gt;
        &lt;domain includeSubdomains="true"&gt;www.example.com&lt;/domain&gt;
        &lt;pin-set&gt;
            &lt;!-- no backup pin declared --&gt;
            &lt;pin digest="SHA-256"&gt;7HIpactkIAq2Y49orFOOQKurWxmmSFZhBCoQYcRhJ3Y=&lt;/pin&gt;
        &lt;/pin-set&gt;
    &lt;/domain-config&gt;
&lt;/network-security-config&gt;
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

You can also visit the
[source code](https://cs.android.com/android-studio/platform/tools/base/+/mirror-goog-studio-main:lint/libs/lint-tests/src/test/java/com/android/tools/lint/checks/NetworkSecurityConfigDetectorTest.java)
for the unit tests for this check to see additional scenarios.

The above example was automatically extracted from the first unit test
found for this lint check, `NetworkSecurityConfigDetector.testMissingBackupPin`.
To report a problem with this extracted sample, visit
https://issuetracker.google.com/issues/new?component=192708.

(##) Suppressing

You can suppress false positives using one of the following mechanisms:

* Adding the suppression attribute `tools:ignore="MissingBackupPin"`
  on the problematic XML element (or one of its enclosing elements).
  You may also need to add the following namespace declaration on the
  root element in the XML file if it's not already there:
  `xmlns:tools="http://schemas.android.com/tools"`.

* Using a special `lint.xml` file in the source tree which turns off
  the check in that folder and any sub folder. A simple file might look
  like this:
  ```xml
  &lt;?xml version="1.0" encoding="UTF-8"?&gt;
  &lt;lint&gt;
      &lt;issue id="MissingBackupPin" severity="ignore" /&gt;
  &lt;/lint&gt;
  ```
  Instead of `ignore` you can also change the severity here, for
  example from `error` to `warning`. You can find additional
  documentation on how to filter issues by path, regular expression and
  so on
  [here](https://googlesamples.github.io/android-custom-lint-rules/usage/lintxml.md.html).

* In Gradle projects, using the DSL syntax to configure lint. For
  example, you can use something like
  ```gradle
  lintOptions {
      disable 'MissingBackupPin'
  }
  ```
  In Android projects this should be nested inside an `android { }`
  block.

* For manual invocations of `lint`, using the `--ignore` flag:
  ```
  $ lint --ignore MissingBackupPin ...`
  ```

* Last, but not least, using baselines, as discussed
  [here](https://googlesamples.github.io/android-custom-lint-rules/usage/baselines.md.html).

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